The Fifth Edition of Magic was the state of the world/magic that existed after Zird the Arcane sacrificed his life using the Hammer of Sundering in Episode 126 - Live Show 2015. This changed the world fundamentally, resulting in a lot of strange changes. Thom the Dragonborn became completely nude, and Jaela became a Paladin. The brief destruction and re-creation of all of magic has some unforeseen consequences, including destroying the plane that the city of Nareev was in. The city was only saved due to the demigod Soloni moving it into the dimension/plane of Drunkeros.
Out of Game
The cast wanted to change the game from a 4th edition game, to a 5th edition game. This was untenable for some time, until they just decided to do it, resulting in a lot of the mechanical changes peppered throughout. I say, it was a long time coming. 5e for life!